AATSR is the latest in a series of instruments designed to measure global sea-surface-temperatures to an accuracy of 0.3 K and to monitor global vegetation coverage and cloud properties. It forms part of the payload on ESA's ENVISAT mission due to be launched in 2000. Features new to AATSR include a different cooler system, an improved mechanical structure, and a corrected visible calibration system, VISCAL. The methods and results of the instrument's pre-launch calibration tests are described. These include the field-of- view, visible and infrared radiometric calibrations. The radiometric responses of the visible/near infrared channels were measured, and the in-flight VISCAL unit was calibrated. The calibrations of the thermal infrared channels were verified over a range of target temperatures between 210 K to 315 K and corrections derived for detector non-linearity. Tests were also performed to verify freedom from any significant scan dependent variations or effects due to changes in the thermal environment. Data from the initial calibration run identified a major fault with the instrument's optical alignment, and was vital in establishing the solution for the eventual repair. In addition, the calibration of the visible channels revealed important characteristics affecting the accuracy of the scientific products that would otherwise have been overlooked.